No heat flow is possible without temperature difference. Thus, the heat flux between air/a gas and a surface of an object will be in direct proportion with the temperature difference between the gas and the surface and with the surface conduction, i.e.φ=h(ta−ts),where φ denotes the heat flux, h the surface conductance, ts the temperature of the surface and ta the temperature of the surrounding gas. Surface conductance is measured in W/m2K.
Heat energy tends to migrate in the direction of the decreasing temperature. The heat transfer can take place by the processes of conduction, convention or radiation. Heat is the energy associated with the perpetual movement of the molecules and temperature is a measure of the vigor of this movement. When materials at different temperatures are in contact the more vigorous molecules transfer some of their thermal energy to less vigorous ones by collisions. This is the process of heat conduction. It is the only way in which heat can flow through an opaque solid.
Thermal energy can be transported through a gas by conduction and also by the movement of the gas from one region to another. This process of heat transfer associated with the gas movement is called convection. When the gas motion is caused only by buoyancy forces set up by temperature differences, then the process is referred to as natural or free convention; but if the gas motion is caused by some other mechanism, such as a fan or the like, it is called forced convection.
For nearly all practically occurring gas flows, the flow regime will be turbulent in the entirety of the streaming volume, except for a layer covering all surfaces wherein the flow regime is laminar (see e.g. 203 in FIG. 2a). This layer is often called the laminar sub-layer. The thickness of this layer is a decreasing function of the Reynolds number of the flow, so that at high flow velocities, the thickness of the laminar sub-layer will decrease.
Heat transport across the laminar sub-layer will be by conduction or radiation, due to the nature of laminar flow.
Concerning radiation all physical objects continuously lose energy by emission of electromagnetic radiation and gain energy by absorbing some of the radiation from other objects that is incident on them. This process of heat transfer by radiation can take place without the presence of any material in the space between the radiating objects.
Concerning conduction the mass transport across the laminar sub-layer will be solely by diffusion. In the technology relating to heat exchangers, it is well known that the principal impediment to the transfer or transmission of heat from a gas to a solid surface is the boundary layer of the gas, which adheres to the solid surface. Even when the motion of the gas is fully turbulent, there exists a laminar sub-layer that obstructs the transmission of heat. While various methods and types of apparatus have been suggested for overcoming the problem such as by means of driving the gas with sonic waves and vibrating the partition with external vibration generators, these methods while being effective to some extend, are inherently limited in their ability to generate an effective minimization of the laminar sub-layer and at the same time covering an area large enough to make the method efficient.
Likewise, the speed of a catalytic process involving a gas reacting with a catalytic surface is, among many things, limited by the interaction between the gas molecules and the catalytic surface, i.e. by the supply of reactants to and the transport of reaction products away from the catalytic surface. The mass transport through the laminar sub-layer covering the catalytic surface can therefore only be done by diffusion of the reactants and reaction products.
Similarly, when one kind of gas or mixture of gases is actively changed to another composition of gases the time needed to flush the inner surface of the container is limited to the time it takes to change the gases in the laminar sub-layer. This change can only be done by diffusion.
Patent specification U.S. Pat. No. 4,501,319 relates to increased heat transfer between two fluids (i.e. not between an object and gas/air) and provides the increased heat transfer by minimizing the thickness of the laminar sub-layer by establishing a standing wave pattern. However, the use of a standing wave pattern to minimize the laminar sub-layer does not give as very efficient or large reduction of the laminar sub-layer (and thereby increase in heat transfer), since the definition of a standing wave pattern includes a stationary and repeatable location of nodes over the surface. At these nodes there will be no displacement or velocity of the gas molecules.
Patent specification U.S. Pat. No. 4,835,958 describes a process for producing work onto rotatable blades of a gas turbine. The described process involves steam as cooling media and a disruption of laminar steam film on the surfaces of a nozzle thereby ensuring increased heat transfer. This is done by establishing a sonic shock wave to disrupt the laminar sub-layer. Since the surface area covered by the shockwave has to be compared to the surface area used to generate the shock wave, the proposed method does not give a reduction of the laminar sub-layer (and thereby increase in heat transfer) over as large an area as the present invention do, since ultrasound disperses over a larger part of the object in question than the shock wave.
Patent specification U.S. Pat. No. 6,629,412 relates to a turbine generator producing both heat and electricity. The description includes a heat exchanger which uses acoustical resonators (formed by cavities in the surface of the heat exchanger) to prevent formation of a laminar boundary layer. The resonators generate acoustic vortices as the gas flows over the surface of the heat exchanger and thereby creating turbulence in the gas over the surface. The generated turbulence will decrease the size of the laminar layer (see FIG. 2a) but the generated acoustic energy is not sufficiently high and therefore not sufficiently efficient at minimizing the sub-layer.
Patent specification JP 07112119 relates to enhancing a catalytic process by applying ultrasound and thereby disturbing a fluid border film over the porous solid catalyst. The arrangement gives an inefficient coupling of the ultrasound from a source/oscillator via the diaphragm and thereafter to the gas. This is related to the large difference in acoustical impedance, which will apply for any solid-gas transition.
Patent specification U.S. Pat. No. 4,347,983 relates to a device for generating ultrasound. It discloses that ultrasound may be useful for enhancing a heat transfer by disruption of a liquid or gas layer. It is further mentioned that catalytic effects can be improved due to molecular breakdown, production of free ions, mixing and other effects. However, this arrangement does not address the disruption of a laminar sub-layer. Further, this arrangement is not very suitable for generating an acoustic pressure at sufficiently high levels needed for effectively disrupting a laminar sub-layer. In addition the causes for improvement of catalytic effects, i.e. molecular breakdown and production of free ions, are effects that only take place under these circumstances in a liquid medium and not in a gaseous medium.